The present invention relates to a method of forming a contact hole in an insulating film or a semiconductor film formed on an insulating substrate. More particularly, the present invention relates to the dry etching method for forming a hole for electrical contact (hereinafter referred to as contact hole) between an electrode of chrome or chrome compound formed on the insulating substrate as an active matrix substrate for a liquid crystal display apparatus and an upper electrode, and relates to the processing method after the dry etching operation.
A matrix type liquid crystal display apparatus is formed to interpose liquid crystal material between a substrate on which TFTs (thin film transistors) made of semiconductor thin films are provided and an opposite substrate, and to apply electrical voltage to the display material selectively for each pixel. Opposite electrodes, color filters, black matrices are on the opposite substrate. The liquid crystal display for which such TFT array substrate is employed is hereinafter referred to as TFT-LCD.
To the TFT array substarate, at least TFTs and pixel electrodes are provided for each pixel in a matrix manner on an insulating substrate made of glass and so on, and further alignment films and storage capacitances if necessary are provided and still further signal lines such as gate lines and source lines are provided.
So as to produce the TFT-LCD, TFTs, gate lines, source lines and other common lines are produced in an array manner to form a display area and input terminals, auxiliary lines and drive circuits are provided on the periphery of the display area. At the same time, so as to obtain their function, conductive films and insulating films are provided if necessary. Further, opposite electrodes, color filters, black matrices are provided on the opposite substrate.
After producing the TFT array substrate and the opposite substrate, keeping a desired gap between two substrates, the two substrates are adhered on the periphery. Thereafter, a liquid crystal material is introduced into the gap and a liquid crystal display is produced.
Various semiconductor devices and solid state devices are provided on the TFT array substrate and on the opposite substrate for a liquid crystal display device, using thin film technology. In such semiconductor devices and solid state devices, semiconductor films, insulating films or conductive films are used.
In a liquid crystal display device, contact holes are provided on necessary portions through interlayer insulating films so as to obtain a contact between a pixel and a gate line; a contact between a pixel and a source line and further between a pixel and a drain line; and a contact between one line and another line. A dry etching method is adopted for forming a contact hole penetrating an interlayer. SF.sub.6 gas, CF.sub.4 gas or mixed gases comprising at least one of SF.sub.6 gas and CF.sub.4 gas, and O.sub.2 gas or inert gases are usually employed for dry etching. Above all, a combination of SF.sub.6 gas and O.sub.2 gas as the most popular gas for the etching rate and selectivity to an underlayer film.
When forming a contact hole, normally the taper shape of the contact hole is controlled by the ratio of photoresist to that of object layer.
In this case, a gas in which fluorine gas and O.sub.2 gas are mixed is employed. If flow rate of O.sub.2 gas is increased, a photoresist is fastly etched and the size of the contact hole becomes large or the photoresist is lost. So, the flow rate of O.sub.2 gas is suppressed lower than that of other mixed gas. And, it is popular that oxygen plasma treatment is conducted after etching operation to remove the plasma damage layer of the photoresist.
Under the etching condition for an insulating film (SiN), a chrome film (hereinafter referred to as a chrome film) can hardly be etched. Some gases are adsorbed on the surface of the chrome film.
Further, etching reaction products adhere to the surface of the chrome film. Therefore, when depositing an ITO film on the contact hole, a preferable electric contact cannot be obtained.
In most cases, an electric resistace value of a contact hole intervening the two films made of the same material, as for example on a chrome film another chrome film is deposited, is low. Further, there are some cases in which resistance value of several kilo-ohms can be applicable for some devices. Thus, these contact holes did not raise a specific problem. However, when we use chrome film line and deposit ITO film on the line and contact through the contact hole, the conventional etching conditions cause extremely high contact resistance not to be used of several megohms. There arises such a neccesity of reducing resistance below several hundred ohms to improve display property.
As a result of researching, it was found that the contact resistance could be particularly high in the case where material of upper layer is different from that of a lower layer, for instance the lower layer is made of a material such as chrome not to be almost etched in the condition of etching on an insulating film, and the upper layer is made of a material such as ITO, aluminum.